1. Field of the Invention
The present invention relates generally to the enhancement and filtering of medical images to remove artifacts. The present invention also generally relates to computerized techniques for automated analysis of medical images, for example, as disclosed in U.S. Pat. No. 5,825,842, which is incorporated herein by reference.
2. Discussion of the Background
In X-ray CT imaging, it is common to low-pass filter or average a number of higher resolution images (“thin” images) in order to reduce artifacts or for other reasons. Typically, the filtering or averaging is performed in the axial direction, which is the direction perpendicular to the plane of the images (i.e., the z direction). The low-pass filtering is referred to as slice thickening because the effective “thickness” of the resulting image slice is larger. The undesirable effect of slice thickening is that image resolution is lost. This includes both resolution of the object of interest (OOI) being imaged, as well as background noise resolution. The effect of reduced noise resolution is that the grain size of the background noise pattern in the z direction is stretched. This is undesirable from a clinical standpoint, especially when viewing images in a coronal or sagittal format.
Images produced from helical multi-slice CT systems exhibit a distinct type of artifact when operated at medium and high helical pitch values. The artifacts appear as alternating light and dark regions around structures whose features change axially. The shape of the artifact is similar to the vanes on a windmill; hence the name “windmill” artifact. This artifact has also been referred to as simply the “helical” artifact in the literature. A common case is a higher density bone structure surrounded by lower density soft tissue. The windmill artifact appears as light and dark alternating “vanes” in the soft tissue emanating from the bone structure. The cause of the artifact is inadequate sampling in the axial direction due to the high helical pitch. This is purely a function of geometry and will occur with all helical multi-slice CT reconstruction algorithms. Images with windmill artifacts are referred to as “artifact” images. Most methods of reducing the windmill artifact also result in smoothing of sharp edges and other structures as well as noise. Such images are referred to as “smooth” images.